The ability to identify lies is centered in the frontal lobe of the brain, which degenerates with frontotemporal dementia, Rankin noted.
This social decline can be an early sign of frontotemporal dementia, the most common form of dementia among people under 65, according to Rankin. Other early signs of the disease can be dramatic changes in behavior or personality.
These changes are often taken as signs of depression or a midlife crisis, and not recognized for the serious condition they represent, Rankin noted.
"We wanted to know if we could use this test [to gain] a better idea of what disease the person has," she said. They discovered they can, she added.
"We want to find these people early," she explained. "We want people to recognize that these social lapses are actually a disease -- parts of their brain are being eaten away."
People who age without suffering neurodegeneration usually do not have a significant loss of their ability to recognize sarcasm and deception, the researchers pointed out.
Dr. Sam Gandy, associate director of the Mount Sinai Alzheimer's Disease Research Center in New York City, said "neuroanatomy can be surprising."
"Phenomena that sound very complex can sometimes be very discretely localized," he added. "Language, short-term memory, and recognition of 'self' are examples of clearly complex phenomena that can be dramatically affected by rather small lesions."
This study provides strong evidence that surprisingly discrete lesions -- in this case, lesions caused by neurodegenerative disorders -- can abolish perception of sarcasm and sincerity, Gandy said.
"The fact that Rankin and colleagues were clever enough to formulate the problem properly so that the study could be undertaken is a testament to their skills as outstanding bedside clinical neurologists," he said.
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